As a fuel injection device, those including a fuel injector that injects the fuel into the cylinder of the internal combustion engine and a control device that controls the opening/closing motion of the fuel injector are known.
The fuel injector includes a needle that opens/closes an injection hole, a control chamber that applies the fuel pressure for energizing the needle to the valve closing direction, and a control valve that controls communication and cutoff between the control chamber and a high pressure port or a low pressure port. The fuel injector increases/decreases the fuel pressure inside the control chamber by making a high pressure fuel to flow out from and flow in to the control chamber, and controls the opening/closing motion of the needle (refer to Japanese Patent No. 3827003: DE-10306296 A1).
In the fuel injector described above, one in which the aperture diameter of the out orifice used for allowing the fuel to flow out from the control chamber to the low pressure fuel path is expanded and the discharge flow rate is increased in order to improve the valve opening responsiveness has been developed.
As an actuator that drives to open the control valve which switches the valve closing state for closing the low pressure port to the valve opening state for opening the low pressure port, a layered piezoelectric element is used. When the layered piezoelectric element that expands/shrinks by charging/discharging of the electric charge is used, such problem as described below occurs in performing injection of a minute injection quantity such as a pilot injection (hereinafter referred to as a minute injection).
To be more specific, the layered piezoelectric element drives to open the control valve by being charged and extending. Thus, the control chamber and the low pressure port communicate with each other, the fuel is discharged from the control chamber, and thereby the needle opens the valve.
On the other hand, the control device sets the injection command value per one injection of the fuel injector from the target injection quantity and the fuel pressure. This injection command value shows the energization time from start of charging to start of discharging with respect to the layered piezoelectric element.
In order to stabilize (maintain) the valve opening state of the control valve, namely the valve opening state of the fuel injector, in the control device, the final target value with respect to the charging voltage for the layered piezoelectric element is set to a value greater than the valve opening starting value that is the charging voltage of the time when the control valve starts valve opening.
Also, the control device starts charging with respect to the layered piezoelectric element based on the injection command value, and, when the charging voltage reaches the valve opening starting value, the control valve opens. Further, when the fuel pressure within the control chamber lowers than the valve opening pressure, the needle opens the valve, and the fuel injection starts.
Also, the control device makes the charging voltage reach the final target value, thereafter subsequently maintains the charging voltage at the final target value to continue the fuel injection, thereafter starts discharging to close the control valve, and the fuel injection finishes.
Further, in the control device, in order that the charging voltage of the layered piezoelectric element reaches the final target value as quickly as possible, the charging period after the charging is started until the charging voltage reaches the final target value is set short, and the charging current is supplied from the drive circuit to the layered piezoelectric element according to this short charging period.
However, when the valve opening responsiveness is improved and the period until the fuel injector starts opening becomes short, the actual injection quantity increases by that portion compared to that of one with low valve opening responsiveness. Therefore, it is necessary to reset the injection command value short so that the actual injection quantity reduces. Particularly, in the minute injection as described above, there is a case where the injection command value becomes extremely small and becomes less than or equal to the charging period.
In such a case, discharging may possibly start before the charging voltage reaches the final target value. As a result, when discharging starts before the charging voltage reaches the final target value, the valve opening behavior of the control valve possibly becomes unstable.
Also, in a minute injection, with the aim of avoiding such situation that the injection command value becomes less than or equal to the charging period and discharging starts before the charging voltage reaches the final target value, it is possible to set the charging period shorter.
However, when the charging period is made shorter, the energization condition in the drive circuit becomes severe, the drive circuit must be replaced with a large current drive circuit, and therefore the cost may possibly increase.